Ball grid arrays (BGAs) are a relatively new semiconductor device which utilize a different manufacturing process than some of the other similar devices such as the quad flat packages (QFPs) and thin quad flat packages (TQFPs). Currently, BGAs are produced using PCB technology which involves a multi-coating process. Inherent in the this multi-coating process is the problem of controlling the thickness of final product. Therefore, unlike QFPs, for instance, where the thickness can be controlled within a tolerance .+-.0.01 mm, BGA specification typically calls for a tolerance as high as 0.1 millimeter. So the maximum variation between the thinnest BGA substrate within the required tolerance and the thickest BGA substrate within the tolerance can be as high as 0.2 millimeters.
This variation in thickness can create various problems for the existing BGA moulding tools during a clamping operation. A typical BGA moulding assembly has a top and bottom cavity bars which make direct contact with the BGA substrate. During a clamping operation, the top and bottom cavity bars are brought together with the BGA substrate positioned in between the bars. This clamping operation is crucial in the BGA fabrication for securing the substrate before resin compound is delivered. Good control over the clamping operation is important to ensure that the substrates are not scratched or damaged during the clamping operation and that sufficient clamping force is applied to minimize leakage of the resins during the moulding operations of the packaging process. Because tight fitting of the top and bottom cavity bars is essential, the current BGA mould assemblies are designed to accept substrates having a narrow range of thickness. This is because when a substrate is too thick, a leakage may result from improper clamping of the top and bottom cavity bars. In addition, the substrate may be irreparably damaged if excessive force is exerted in an attempt to fully mate the cavity bars.
Currently, virtually every BGA moulding system uses a dual in-line system where two sets of BGA substrates are encapsulated at the same time. This requires two identical sets of cavity bars which must be placed near each other side by side. Both of the cavity bars are controlled by single driving mechanism, e.g., hydraulic press, and so each bar cannot work independent of each other. This limitation is particularly problematic when the two substrates being encapsulated are of different thickness. Since the press has no way of accounting for the difference in thickness, it attempts to exert the same amount of force on both. Consequently, one of the substrates is over-cramped and damage occurs.
To prevent the over-cramping from occurring, springs were added to the cavity bars with each bar being able to exert different amount of force to its respective substrates. As FIG. 1 demonstrates, the amount of force being exerted onto the substrates is wholly depended on the compression of the springs which in turn is depended on the thickness of the substrate. So where a pair of substrates has different thickness, different amount of force is exerted onto each substrate. Note that the springs in the left mould assembly are much more compressed than the springs in the right mould assembly.
In other situations, a pair of substrate may have the same thickness. However, their thickness may vary from the thickness of a pair from a different batch, as shown in FIGS. 2A and 2B. Note that the substrates in FIG. 2B are much thinner than the substrates in FIG. 2A, and hence, their corresponding springs of FIG. 2B are much less compressed than the springs of FIG. 2A. Therefore, the substrates in FIG. 2A experience greater clamping force than the substrates in FIG. 2B.
It can be seen from these illustrations that there is no control over the clamping force in the current BGA moulding systems, and that the force varies from substrate to substrate. The result is that in some cases, the clamping force is excessive; in other cases, the force is inadequate. Where there is excessive force, the substrate is damaged; where not enough force is exerted, there can be leakage of a resin material.
Despite these problems, the spring-buffered cavity bars are the standard in virtually all of the current BGA moulding systems, and while many BGAs fail to meet the performance specification, the industry has failed to provide a BGA moulding system which can eliminate or effectively reduce these problems. Indeed, to the best of the inventor's knowledge, no BGA mould assembly is currently available which can consistently accommodate BGA substrates having a wide range of thickness, simultaneously handle a pair substrates having different thickness, and which can precisely control the amount of force being exerted onto each substrate. Clearly, there is a need in the industry for a BGA moulding assembly which overcomes these shortcomings to provide to an assembly which can controllably exert the right amount of clamping force to each substrate.